1. Field of the Invention
This invention generally relates to a projection optical system, and specifically relates to a projection optical system for photolithography used in producing integrated circuits, large-scale integrated circuits, or the like.
2. Description of the Prior Art
Conventional projection optical systems are of the refraction type and the reflection type. Japanese published examined patent application No. 57-12966 and Japanese published examined patent application No. 55-10883 disclose projection optical systems of the refraction type. Japanese published unexamined patent application No. 55-17196 and Japanese published examined patent application No. 28-5735 disclose projection optical systems of the reflection type. The conventional projection optical systems are unsatisfactory in a few points as described hereinafter.